The present invention relates to a multibeam klystron and particularly to a multibeam klystron exhibiting a high bandwidth and substantial power output at high frequencies.
Klystrons can be used both as amplifiers of microwave energy and as oscillators. They consist of three elements: an electron gun which generates a pencil-like flow of electrons accelerated to high energy, a microwave interaction region where the energy in the electron beam is converted to microwave energy, and finally a collector to collect the spent electrons and recover energy that remains. In the aforementioned interaction region the electron beam passes through the center of an r.f. excited toroidal cavity employed for the purpose of accelerating and decelerating electrons in the electron beam at the rate of the r.f. energy in the cavity. The electron beam is then directed via one or more intermediate cavities, to an output cavity where amplified r.f. energy is withdrawn. In the region along the electron beam between the input and output cavities, “bunching” of the electron beam takes place, this effect being enhanced by the intermediate cavities. The bunching of the electron beam produces a strong r.f. field in the output cavity.
The conventional single beam klystron has a disadvantage in that it amplifies only over a relatively narrow band of frequencies due to the high Q of the microwave cavities. A further problem associated with conventional single beam klystrons is that the single beam must have relatively high perveance and DC electron density to provide enough beam power to produce substantial microwave output. High perveance and electron density mean high repulsive forces between electrons in the beam, causing bunching to be inhibited whereby the desired current density variations in the beam are limited.
Most klystrons employ a heavy magnetic structure for the purpose of focusing the comparatively high power beam. The structure may comprise permanent magnets or electromagnets that in any case account for substantial weight and large size for an otherwise relatively small electronic device. These weight and size factors, as well as the undesirable surrounding magnetic field, are limiting factors for a number of uses of the device. Electrostatic focusing proposed for single beam klystrons is not suitable for most high power applications and in any case proposals for electrostatic focusing have involved the same heavy klystron envelope structure utilized for permanent magnets or electromagnets.
If, instead of employing but a single beam, several parallel electron beams were to be used, then each beam can have lower perveance and thus provide large current variations, but in the aggregate, the total power output can be comparatively high. An ancillary benefit would be lower voltage for a given power. Further advantages of a multibeam structure are higher basic efficiency and wider bandwidth. However, multibeam klystrons as heretofore proposed utilize magnetic focusing to constrain the electron beam. Magnetic focusing of a plurality of beams is difficult because some of the beams must be off the center axis of the tube, i.e., the symmetry axis of the magnetic field is not the same for all the beams. All but one of the electron beams must be off the symmetry axis of the magnetic field of the klystron. Other beams must cross magnetic field lines and, in so doing, they are defocused. Complex magnetic systems have been proposed in an attempt to correct this problem, but add to the expense and weight of the device.